INFECTIOUS DISEASES. 295 



permanent habitat in Europe appears to have been a limited area 

 in the southeastern portion, from which it occasionally spread north- 

 ward, without, however, extending much beyond the limits of the 

 Balkan peninsula. During the early part of the present century it 

 still occurred to some extent in this region, where it prevailed as an 

 epidemic for the last time in 1841. 



Typhus fever, like smallpox, is a disease which is transmitted by 

 personal contagion, and its dissemination depends upon human inter- 

 course. It prevails chiefly in temperate or cold regions, and is un- 

 known in the tropics except at considerable elevations above the sea 

 level. In temperate regions its season of greatest prevalence is the 

 winter and spring. There is no reason to suppose that the specific 

 germ, which has not yet been demonstrated, is able to multiply ex- 

 ternal to the bodies of infected individuals, and, consequently, condi- 

 tions relating to soil, moisture, temperature, and organic decomposi- 

 tion are apparently without influence in the development of the 

 disease, except in so far as they affect the predisposition of those 

 exposed to infection. Insanitary surroundings no doubt constitute 

 a predisposing cause by lowering the vital resisting power of those 

 exposed to such influences. But of all the predisposing causes war 

 and famine are shown by the history of past epidemics to have been 

 the most potent. 



The earliest reliable accounts of epidemics of this disease date 

 from the eleventh century, but it was not until the sixteenth century 

 that well-recorded accounts of the epidemic prevalence of the disease 

 were made, in the first instance by Italian physicians. The disease 

 prevailed extensively in Italy during the years 1505 to 1530. In the 

 seventeenth century numerous fatal epidemics occurred in various 

 parts of Europe, the disease for the most part following in the track 

 of contending armies, and adding to the scourge of war with its dev- 

 astations and the resulting scarcity of food the disastrous effects 

 of a deadly pestilence. During the eighteenth century the dis- 

 ease continued to prevail in Europe, and three notable epidemics 

 occurred in Ireland: the first in 1708 to 1710, the second from 

 1718 to 1721, the third from 1728 to 1731. The last two epi- 

 demics, although most destructive of life in the famine-stricken 

 districts of Ireland, also extended to a considerable portion of Eng- 

 land and Scotland. In 1734 to 1744 typhus prevailed extensively 

 in eastern and central Europe; it again obtained wide prevalence 

 in 1757 to 1775, a period of wars and famine, and during the last 

 ten years of the eighteenth and the early part of the present cen- 

 tury, the period of the Napoleonic wars, it again ravaged the coun- 

 tries over which the contesting armies passed. Ireland appears to 

 be one of the endemic foci of this disease, and when it has invaded 



